Dysfunctions of the cerebral cortex, the highest-level processor of our sensations, perceptions, and decisions, are thought to underlie numerous neurological and psychiatric disorders. A major obstacle to treating such pathology is the high degree of complexity of cortical circuitry, which has remained largely enigmatic. Each primary sensory area of cortex receives connections from a primary and secondary thalamic nucleus. Strokes destroying primary thalamic nuclei cause near complete loss of sensation, but damage to secondary thalamus produces complex behavioral deficits. Secondary thalamus is also connected with diverse cortical areas, suggesting a possible role in psychiatric disorders. Nevertheless, the functions of secondary thalamus remain unknown. Are secondary nuclei alternate sensory pathways to cortex? Or key sources of behavioral signals? This project will focus on the secondary somatosensory thalamus (the posterior medial nucleus, POm). POm has sometimes been regarded as an afferent sensory pathway, operating in parallel with the primary thalamic relay, but several studies support the view that POm is instead downstream of primary somatosensory cortex. POm has been suspected of mediating communication between cortical areas and, more recently, to aggregate motor signals to provide as feedback to cortical areas. Like secondary visual thalamus, POm may also have a role in selectively enhancing particular stimuli. The first goal of this project is to investigate coding of sensory, motor and behavioral signals in POm. The second goal is to understand their functional impact on primary somatosensory cortex, particularly on apical dendrites in cortical layer 1-a major target of POm. The third goal is to determine whether or not simple sensory behaviors require POm. To achieve these goals, we will combine mouse behavior with electrophysiology, two-photon microscopy, and optogenetics. We aim to identify the role of this long obscure circuit component in somatosensation. Identifying fundamental functions of this secondary thalamic nucleus will likely pave the way for future studies in other neocortical systems and in higher-order species.